POLYOLEFIN POROUS MEMBRANE AND METHOD OF PRODUCING THE SAME

A method for producing a polyolefin-based porous film includes an (A) step: a raw fabric forming step for forming a non-porous raw fabric from a polyolefin-based resin composition, a (B) step: an MD cold stretching step for cold stretching the non-porous raw fabric obtained in the (A) step at a temperature of -20°C to (Tm-30)°C (Tm is a melting point (°C) of the non-porous raw fabric) in an extruding direction (MD) of the raw fabric to make the raw fabric porous; a (D) step: a TD cold stretching step for cold stretching a film processed in the (B) step in a direction (TD) perpendicular to the MD, and an (H) step: a thermal fixing step, in the above order. A thermal fixing temperature T 2 (°C) in the (H) step satisfies expressions (1) and (2) below; T 2 > T 1 (1), (Tm-3) ¥ T 2 ¥ (Tm-40) (2) (in the expressions, T 1 represents a stretching temperature (°C) in the (D) step, and Tm represents the melting point (°C) of the non-porous raw fabric). A polyolefin-based porous film made of a polyolefin-based resin composition has a pore surface area of 35.0 to 42.0 m 2 /g and birefringence of 2.0×10 -2 to 4.0×10 -2 .